Electrophotographic printing is a non-impact printing technology invented by Chester Carlson in the 1930s. It occupies a large segment of the total printing market, with a global market value of $59.9 billion in 2009. Electrophotographic printing is a highly complex printing technology consisting of 2 core components, namely the photoconductor (PC) and the toner. The printing process involves 7 distinct steps, which include PC charging, PC exposure, toner development, toner transfer, fusing, cleaning, and charge erasure. The photoconductor, as a primary component, is involved in 6 of the 7 aforementioned steps. Thus, both photoconductor durability and performance are required and highly sought-after characteristics.
The photoconductor is the component through which a latent image can be formed, with the latent image being developed by toner particles in the subsequent step. Initially, an electrostatic charge is distributed through projection on the surface of the PC. Next, light exposure results in generation of charge carriers within the PC and through absorption of light by the Charge Generation Material (CGM). The charge carriers are transported to the PC surface and the opposite electrode by Charge Transport Material (CTM). As the charge carriers reach the surface, they neutralize surface charges within the area previously illuminated. This forms a latent image on the surface of the PC, which can then be subjected to toner development.
Photoconductor performance relies on several factors, including charge acceptance during projection of charge on PC surface, free charge generation and transport following illumination, and the degree of surface charge neutralization. All these factors work in concert to exemplify the overall performance of a photoconductor.
The performance is typically measured in terms of sensitivity of the photoconductor to light exposure at a particular wavelength, with higher photosensitivities associated with enhanced PC performance.
The charge generation material (CGM) incorporated in a photoconductor serves the above-mentioned photoconductor performance. Desired CGM characteristics include efficient absorption of light at the exposure wavelength, low recombination of initially-generated charges, the ability to produce free charges and transfer charges to transport material, and photostability. As such, photoconductor performance relies on both the optical/electronic properties of the CGM and manipulation of these properties through the choice of correct material and environment. In addition, it is desirable to manufacture PC in a cost-effective manner, so as to reduce the overall cost of the printing device.